. .
.
Simulating a WiFi Network
.
.

 

Objectives

 

After completing this experiment you will be able to:

 

  • Understand about Wi-Fi network and its’ different standards, protocols and requirements for connecting a Wi-Fi network
  • Identifying the different issues related to hidden terminal problem and exposed terminal problem; also they can solve and simulate with the open source network simulator NS2/NS3
  • Analyze the Wi-Fi communication range in the presence of the access point (AP) and the base station (BS)

 

Time Required

 

Around 3.00 hours

 

WiFi Network

 

Wi-Fi is stands for wireless fidelity. Wi-Fi uses the 802.11 standard; it was developed by the Institute of Electrical and Electronics Engineers (IEEE) in 1997. Wi-Fi was superseded by the extensions 802.11a and 802.11b, and later by 802.11g. Wi-Fi technology uses radio communication and operating at a frequency of 2.4GHz. [v]

 

What are IEEE 802.11 standards?[vi]

 

  • 802.11 is the original wireless local area networks standard. It supports 1 Mbps to 2 Mbps.
  • 802.11a is a very high speed wireless local area networks standard for 5 GHz band and it supports 54 Mbps. It uses an orthogonal frequency division multiplexing (OFDM) encoding scheme.
  • 802.11b is a wireless standard for 2.4 GHz band. It supports 11 Mbps. 802.11b uses only DSSS( Direct Sequence Spread Spectrum).
  • 802.11d is a international roaming. This automatically configures devices to meet local RF regulations.
  • 802.11e address the quality of service requirements for all IEEE wireless radio interfaces.
  • 802.11f defines inter-access point communications to facilitate multiple vendor-distributed wireless local area networks.
  • 802.11g establishes an additional modulation technique for 2.4 GHz band. This supports speeds up to 54 Mbps.
  • 802.11h defines the spectrum management of the 5 GHz band.
  • 802.11i address the current security weaknesses for both authentication and encryption protocols.
  • 802.11n provides higher throughput improvements. It also provides speeds up to 500 Mbps.

 

The basic difference between 802.11a,802.11b and 802.11g are given below :

 

Parameter

802.11a

802.11b

<802.11g

Standard approved

Sept 1999

Sept 1999

June 2003

Available bandwidth

300MHz

83.5MHz

83.5MHz

No. of overlapping channel

4

3

3

Frequency

5GHz

2.4GHz

2.4GHz

Typical Data Rate

23 Mbit/s

4.5 Mbit/s

19 Mbit/s

Maximum Data Rate

54 Mbit/s

11 Mbit/s

54 Mbit/s

Range

115 feet

115 feet

125 feet

Compatibility

None

None

backward compatible with b

Advantages

·         fast maximum speed

·         regulated frequencies prevent signal interference from other devices

 

·         lowest cost

·         signal range is good and not easily obstructed

·         fast maximum speed

·         signal range is good and not easily obstructed

Dis-advantages

·         highest cost

·         shorter range signal that is more easily obstructed

·         slowest maximum speed

·         home appliances may interfere on the unregulated frequency band

 

·         costs more than 802.11b

·         appliances may interfere on the unregulated signal frequency

 

 

 Hardware Requirements for Wi Fi

 

The following hardware devices are required for connecting the Wi Fi Network.

 

Access Point

 

Access Point acts as a bridge between the wireless devices and wired network. It allows multiple devices to connect through it for accessing the network. An AP can also act as a router by which the data transmission can be possible from one access point to another.

 

Wireless Network Card

 

A wireless network card is required on each device on a wireless network. A laptop usually has an expansion slot which the network card would fit in to. A desktop computer would need an internal card, which will usually have a small antenna or an external antenna on it. These antennas are optional on most equipment and they help to increase the signal on the card.

 

Transmitter

 

A transmitter is basically used for emitting the wireless signals and it also receive the connection requests where a wireless client will send the requests and receives the replies from the transmitter. Here the transmitter is the wireless router.

 

How to connect Wi-Fi Network?

 

Wi-Fi Network is very easy to connect. If you will think about your laptop with any operating systems, then you can easily connect to a Wi-Fi network for accessing or you can share different files on a network.

 

Once you have acquired the necessary wireless networking hardware then the next step is to connect it all together to form a network and allow each device to communicate. The instructions below will act as basic guidelines of what needs to be done.

 

  • The distance between each computer should be below 100 meters
  • Each computer should be on the same floor
  • Plug the access point into the power outlet and existing Ethernet jack on the network
  • Configure the access point (usually via a web browser)
  • Configure the client computers with the appropriate network settings required to be able to communicate with the access point.
     

 

Benefits of Wi Fi

 

Following are the different benifits of Wi Fi Networks

 

  • In wireless ad-hoc network mode, devices like consumer electronics and gaming applications can directly connect and exchange data with each other.
  • Digital images can be transferred wirelessly from cameras and other devices.
  • All connected devices within the range have access to internet and inter-networking.
  • Wi-Fi enables wireless voice-applications (Vo WLAN or WVOIP).
  • Wi Fi provides a secure computer networking gateway, firewall, DHCP server and an intrusion detection system among many other features.
  • Cost of cabling and network deployment of Local Area Networks is significantly reduced.
  • Can be used at placed where wiring and cable lay-out is not feasible
  • Due to its cost effective nature, it can be used widely in corporate set-ups and different educational campuses.
  • A standard Wi Fi device can function in any geographical location.

 

Limitations

 

Like any other types of technology, Wi Fi has its set of drawbacks that are listed as follows:

 

  •  Global inconsistency of spectrum assignments and operational limitations.
  • Overlapping of channels
  • Limited range of equivalent isotropically radiated power in some areas.
  • Greater power consumption compared to lower bandwidth standards.
  • Limited battery life due to range and reach requirements.
  • Wi Fi network range is also limited.
     

 

MAC Protocols

 

The 802.11 standards use a MAC layer known as CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance).

 

In CSMA/CA a Wireless node that wants to transmit & performs the following sequence:

 

  1.  Listen on the desired channel.
  2. If channel is idle (no active transmitters) it sends a packet.
  3. If channel is busy then, the node waits until the transmission end then a contention period where minimum time a host must transmit before it can be sure that the no other host’s packet has collided with its transmission.
  4. If the channel is still idle at the end of the contention period, then the node transmits its packet otherwise it repeats the process defined in step-3 above until it gets a free channel.

 

The MAC header format shown in the figure-01 below: [vi]

 

 

How RTS/CTS exchange Data?

 

 

Step 1:

 

At first the sender check whether the medium is idle or not, if so, after the Distributed Inter Frame Space (DIFS will check the status and sense before transmitting the data in the wireless medium) units of time, it will broadcasts a Request-to-Send (RTS) frame to the receiver address.

 

Step 2:

 

If the receiver is within the range, then it will wait for Short Inter Frame Space (SIFS is the small time interval between the data frame and its acknowledgment) unit of time, then only it will respond to the sender with a Clear-to-Send (CTS) frame.

 

Step 3:

 

If the sender receive the CTS frame, then it will wait for another SIFS unit of time before sending the data frame to the receiver.

 

Step 4:

 

Finally, when the receiver will successfully receive the data frame, it will wait for SIFS unit of time and also send an Acknowledgement (ACK) message return to the sender.

 

Following figure-02 shows howData exchanges using RTC/CTS

 

Figure-02:How Data exchanges using RTC/CTS

 

Issues in WiFi networks

 

  •  Hidden Terminal Problem
  • Exposed Terminal Problem
     

The Hidden Terminal Problem

 

The hidden node/ terminal problem found at a point to multipoint network and it is defined as being one in which three or more nodes are present. Let there are three nodes :node A, node B and node C.

 

A and C cannot hear each other.

 

A sends to B, C cannot receive A.

 

C wants to send B, C senses a free medium.

 

Collision occurs at B.

 

A cannot receive the collision.

 

A is hidden for C.

 

The following figure-03 shows the Hidden Terminal Problem using node A,B and C

 

Figure-03: Hidden Terminal Problem

 

Solution of Hidden Terminal Problem.

 

The solution of hidden terminal problem is as follows.

 

  • When A wants to send a packet to B , A first sends a Request-to-send (RTS) to B.
  • On receiving RTS, B responds by sending Clear-to-Send (CTS)
  • When C overhears a CTS, it keeps quiet for the duration of the transfer.
  • Transfer duration is included in both RTS and CTS.
  • RTS and CTS are short frames, reduces collision chance.

 

The other methods that can be employed to solve hidden terminal problem are :

 

- Increase Transmitting Power From the Nodes.

 

- Use unidirectional antennas.

 

- Remove obstacles.

 

- Move the node.

 

- Use protocol enhancement software.

 

- Use antenna diversity.

 

Effect of Hidden Terminal Problem

 

If one node hidden to another then the re-transmission will increase. It also increase the delay and decrease the throughput.

 

Exposed Terminal Problem.

 

Suppose there are four nodes: node A, node B, node C and node D.

 

Here -

  • B can send to both A and C
  • C can send to D, but not to A or B.
  • A and C cannot hear each other.

 

Now the Problem as follows -

 

- When B transmits to A, C detects the transmission using the carrier sense mechanism.

 

- So C defers transmitting to D.

 

- But C could have sent to D, so blocked unnecessarily.

 

The following figure-04 shows the Exposed Terminal Problem using node A,B,C and D

 

 

Figure-04: Exposed Terminal Problem

 

Solution to Exposed Terminal Problem

 

The solution of exposed terminal problem is as follows.

 

  • Suppose B sends RTS to A.
  • A sends CTS to B.
  • C hears RTS, but not CTS, assumes it is ok to send to D.

 

Cite this Simulator:

.....
..... .....
Copyright @ 2017 Under the NME ICT initiative of MHRD (Licensing Terms)
 Powered by AmritaVirtual Lab Collaborative Platform [ Ver 00.12. ]